The present invention relates generally to the measurement of battery current and more particularly to a system which accurately determines the current of the battery from a frequency signal generated in response to the battery current.
In order to fully charge a battery, the current of the battery must be monitored precisely to determine the rate of charge to be applied to the battery. Specifically, in a pack batteries, the charging rates for each of the batteries may be different due to the physical differences between the batteries. One of the batteries may be fully charged, while another one may be only partially charged eventhough both batteries were given the same amount of charging current and charging time.
In order to fully charge all of the batteries, there is provided an equalization mode at the end of the charging cycle. The equalization mode typically begins when the batteries are about 95% charged. The current to the batteries is reduced during the equalization mode in order to provide a slow charge which tops off the charge of the batteries. The current provided to the batteries during the equalization mode may be from about 0.5 amps to 2.0 amps. It is critical that the current be monitored accurately during the equalization mode in order to equalize the charge of all batteries in the battery pack.
Typically, a Hall sensor has been used to monitor the current provided to the batteries during the equalization mode. The Hall sensor may be error compensated to read the current flowing to the battery at a prescribed value (i.e., around 1 amp). However, often during the equalization mode, the current provided to the batteries may differ from the prescribed value depending upon how much the battery must be charged. As previously mentioned, the current may vary anywhere from about 0.5 to 2.0 amps during the equalization mode which is out of the error compensation range for the Hall sensor. Accordingly, the prior art current measurement systems could not accurately determine the current of the battery over a wide range. As will be recognized, the current through the battery may vary as much as xc2x1350 A for high power applications.
With the need for vehicles to become more environmentally friendly, automakers are creating hybrid, mini-hybrid, fuel cell and pure electric vehicles which utilize a battery as an energy storage device. In the fuel cell and pure electric vehicle, the battery powers electric motors which propel the car. The batteries are charged either through regenerative braking and/or an outside source of electrical power such as the fuel cell. Hybrid and mini-hybrid vehicles do not use an outside source of power to charge the batteries. In hybrid vehicles, a small engine is used to charge the batteries and propel the vehicle. Electric motors are also used to propel the hybrid vehicle. As such, the battery pack is relatively large and provides around 300 volts. Mini-hybrid vehicles use a small battery pack having about 42 volts that is used to power an integrated starter generator of the conventional engine. In both hybrid and mini-hybrid vehicles, the battery is charged via the engine and regenerative braking.
It will be recognized that it is imperative to fully monitor the current during the charging of the batteries of the pure electric, fuel cell hybrid and mini-hybrid vehicles. In order to improve the life expectancy of the batteries, and decrease service and replacement thereof, the current of each of the batteries should be closely monitored in order to avoid overcharging and/or undercharging. Having a fully charged battery pack wherein all of the batteries are charged at an equal capacity will increase the efficiency of the electric vehicle and the life of the battery pack.
The present invention addresses the above-mentioned deficiencies in the prior art current measurement systems by providing a system and method which accurately measures the current of each of the batteries of a battery pack. More specifically, the present invention provides a system which is capable of providing a highly accurate measurement of the battery current during the equalization process. The system of the present invention is operative to provide an accurate current measurement for a wide-range of battery current.
In accordance with the present invention, there is provided a current measurement system for measuring the current flowing through a battery. The system includes a shunt resistor in electrical communication with the battery and operative to generate a shunt voltage in response to the current flowing through the battery. The system further includes a voltage to frequency converter in electrical communication with the shunt resistor. The voltage to frequency converter is operative to generate a frequency signal from the shunt voltage. A processor is in electrical communication with the voltage to frequency converter. The processor is operative to determine the current flowing through the battery in response to the frequency signal generated by the voltage to frequency converter such that an accurate measurement of battery current can be made.
The processor may be programmed with at least one value of the frequency signal and an associated value of the current flowing through the battery. In the preferred embodiment, the processor is programmed with multiple values for the frequency signal and associated multiple values of the current flowing through the battery. In this regard, the processor may be programmed to linearly interpolate the value of the current flowing through the battery from the multiple values of the frequency signal and associated current values. In the preferred embodiment, the processor is programmed with eleven data points corresponding to the current flowing through the battery and eleven values of frequency associated therewith.
In accordance with the present invention, there is provided a method of measuring the current flowing through a battery. The method begins by generating a voltage in response to the current flowing through the battery with a shunt resistor. Next, the voltage is converted into a frequency signal with a voltage to frequency converter. A processor will then determine the current flowing through the battery from the frequency signal and a table associating a value of the frequency signal with a value of the current flowing through the battery. In this regard, the processor will determine the current flowing through the battery by finding the current value associated with the value of the frequency signal from the voltage to frequency converter. The processor can also linearly interpolate the value of the current flowing through the battery from the value of frequency and the associated current flows stored in the table. Accordingly, the processor is operative to make an accurate determination of the battery current.